1. Hello world-level Boot Code

The OS development environment has been set up in the previous section. Now let's write a simple guide Code Right

First, let's talk about the operating system boot process:
After the PC power-on self-check, the content of 0 head, 0 track, and 1 Sector of the boot device will be loaded to the memory 0x7c00, and then the Boot Code will be directed to it. I just briefly said, please google for details. Boot. s
1 . Code16
2 . Text
3 . Globl _ start:
4 _ Start:
5 CLI
6 Ljmp $ 0x7c00 , $ 0
7 Go:
8 Movw % CS, % Ax
9 Movw % Ax, % DS
10 Movw % Ax, % SS
11 Movw $ 0xff00 , % SP
12 STI
13
14 Movw $ Zero X 0007 , % BX
15 Movw $ 16 , % CX
16 Movw $ 0 , % DX
17 Movw $ MSG, % BP
18 Movw $ Zero X 1301 , % Ax
19 Int $ 0x10
20
21 JMP.
22 MSG:. ASCII " Hello, OS world! "
23 . Org 510
24 . Word 0xaa55

Boot. s uses the at&t format Assembly syntax. The task is to set the value of the relevant segment register first, and then call the 10 h interrupt display string

To load it to bochs, we also need to use make to compile it into pure binary code and write the corresponding configuration file. Makefile
1 As = As
2 LD = LD
3 Objcopy = Objcopy
4  
5 ALL: floppy. img
6
7 Floppy. IMG: boot
8 Dd If =/ Dev / Zero / Of = Floppy. IMG BS = 1024 Count = 1440
9 Dd If = Boot = Floppy. IMG BS = 512 Count = 1 Conv = Notrunc
10  
11 Boot: boot. o
12 $ (LD) - N - E _ start - Ttext 0   - O $ @. Elf $ ^
13 $ (Objcopy) - S - O binary $ @. Elf $ @
14  
15 Boot. O: boot. s
16 $ () - O $ @ $ ^
17
18 Clean:
19 Rm - RF * . O * . Img * . Elf bochs. Log boot

Google 1 Megs: 32
2 Romimage: File = $ Bxshare / Bios - Bochs - Latest #, address = 0xf0000
3 Vgaromimage: File = $ Bxshare / Vgabios - Lgpl - Latest
4 Floppya: 44 = Floppy. IMG, status = Inserted
5 Boot: